Junichi Inaba, Bo Min Kim, Yan Zhao, Qi Huang, Wei Wei
Phytoplasmas are unculturable, phloem-restricted bacterial pathogens responsible for devastating diseases in crops and ornamentals worldwide. Their mechanism for nutrient acquisition from host plants remains largely unknown. This study demonstrated that infection with potato purple top (PPT) phytoplasma induced extensive remodeling of lipid metabolism in tomato plants, closely linked to autophagy activation. Western blot and confocal analyses revealed increased ATG8 lipidation and autophagosome formation at ER stress sites, alongside the redistribution of lipid droplets (LDs) toward phytoplasma cells. Lipidomic profiling showed a decline in chloroplast galactolipids and phospholipids with a concomitant rise in triacylglycerol (TAG), indicating accelerated membrane turnover and neutral lipid sequestration. Transmission electron microscopy further revealed frequent spatial proximity between LDs and phytoplasmas. Inhibition of autophagy with 3-methyladenine blocked LD breakdown, disrupted ER organization, and reduced phytoplasma titers, suggesting that host autophagy contributes to phytoplasma proliferation. In addition, genome analysis identified a conserved phytoplasma-encoded alpha/beta hydrolase (PPT-lipase), predicted to be related to monoacylglycerol lipases. In vivo assays in yeast and Nicotiana benthamiana confirmed that PPT-lipase reduced neutral lipids, mainly TAG, and that catalytic triad mutations abolished activity. Because PPT-lipase lacks a predicted secretory signal peptide, it likely functions intracellularly within phytoplasma cells and may participate in the metabolism of lipid intermediates. These findings support a model in which phytoplasma infection is associated with host autophagy-associated lipid droplet mobilization and a phytoplasma lipase that may contribute to host-derived lipid resources, providing insight into potential nutrient acquisition strategies of phloem-restricted pathogens.